These two together shouldn't be a problem: I've been taking them in much higher doses for quite a while now. In fact, Methadone is used regularly by pain management doctors as a "booster" for other pain killers in your regimen and helps your baseline drugs to be more effective.

Currently, I take 100mg of Methadone a day (2 x 10mg x 5 times daily) along with 240mg of Oxycontin (80mg x 3 times daily) for my baseline pain and Norco for breakthrough pain, and I have no problems whatsoever.